The interior of large commercial buildings, such as supermarkets, warehouses, department stores, office buildings and the like are often naturally illuminated during some part of a day by sunlight. During periods of darkness or reduced sunlight, artificial light is usually provided.
To take advantage of the sunlight, the artificial light is typically provided using a plurality of lighting circuits which can be individually controlled. As the intensity of the sunlight changes in a predetermined area in the interior of the building, the lighting circuits used to illuminate the area are turned on and off as needed. By turning the lighting circuits on and off as needed, both money and energy are conserved.
Heretofore, the turning on and off of individual lighting circuits in the interior of a building wholly or partially illuminated by sunlight has been done manually or with the use of photocells separately or in combination with a settable timing apparatus, such as clock-driven switches. In large buildings such methods and apparatus have frequently proven to be time-consuming and costly. For example, in the case of manually controlled systems, during cloudy or stormy days, there is a tendency for maintenance personnel to turn the lights on and leave them on even though the ambient natural light intensity in the area artificially illuminated may increase to adequate natural light levels during periods of the day. In the case of automatic systems, there is frequently little inclination to readjust clock-driven switches as the days get longer and shorter throughout the year. Moreover, even when using clock-driven switches, there is still a need for a manual or some type of photocell-controlled switching apparatus to turn the lights on and off during unexpected periods of natural darkness and light.
In a conventional lighting system using a photo-cell, to turn a light on and off, it has been found that, unless the photocell is shielded from the artificial light which it controls, or some other precaution is taken, the turning on of a light will activate the photocell to turn the light off. Thus, it has been the practice to place the photocell in a position wherein it only responds to natural light intensity and is not affected by the light which it controls. This practice, however, does not provide adequate lighting control in areas in which the ambient light intensity changes independently of exterior or natural light intensity. For example, in a warehouse, the placing of boxes or the like in front of a window could significantly reduce interior light intensity without affecting the intensity of the light sensed by an exterior photocell.